1. Field of the Invention
The present invention relates to an apparatus for and a method of capturing a radiation image of a subject by applying a radiation emitted from a radiation source to the subject and detecting the radiation that has passed through the subject with a radiation detector.
2. Description of the Related Art
In the medical field, there have widely been used radiation image capturing apparatus which apply a radiation emitted from a radiation source to a subject and detect the radiation that has passed through the subject with a radiation detector.
One known radiation detector for use in the radiation image capturing apparatus includes a solid-state detector in a laminated structure comprising a matrix of charge collecting electrodes disposed on an insulating substrate and a radiation conductor disposed on the charge collecting electrodes for generating electric charges depending on the radiation that is applied. The electric charges generated by the radiation conductor and representing radiation image information are collected by the charge collecting electrodes and temporarily stored in an electric storage unit. The collected electric charges stored in the electric storage unit are converted into an electric signal, which is output from the solid-state detector. Other known radiation detectors include a radiation detector comprising a charge-coupled device (CCD) and a radiation detector comprising a combination of amorphous silicon and a scintillator.
The radiation image capturing apparatus are required to acquire high-quality radiation images. The radiation detector that is used in the radiation image capturing apparatus is highly sensitive and hence is highly susceptible to the temperature of the environment in which it is used. While the radiation image capturing apparatus are in operation, offset data of the radiation detector may vary, tending to adversely affect the radiation images that are acquired. Furthermore, since a high voltage is applied to the solid-state detector of the radiation detector, the acquired radiation images may be altered due to fluctuations of the voltage. The offset data may also vary due to a time-dependent change in the sensitivity of the solid-state detector.
Japanese laid-open patent publication No. 2003-578 discloses a process of correcting variations of the offset data.
According to the disclosed process, as shown in FIG. 7 of the accompanying drawings, a radiation detector 2 has non-exposure areas 6 on the respective corners thereof which comprise sheets of a material such as lead or the like that is impermeable to a radiation X emitted from a radiation source 4. A radiation image of a subject 10 which is recorded in an exposure area 8 of the radiation detector 2 is corrected using reference data obtained from the non-exposure areas 6 and dark-image offset data obtained from the exposure area 8 when it is not irradiated with the radiation X.
The radiation image obtained from the exposure area 8 is adjusted by the dark-image offset data, and a variation of the radiation image due to a temperature change, etc. is corrected by the reference data.
According to the disclosed process, however, if the state of the apparatus at the time the dark-image offset data are acquired and the state of the apparatus at the time the radiation image is captured are different from each other, then since the dark-image offset data are lowered in reliability, the radiation image cannot be corrected highly accurately. For correcting the radiation image highly accurately, it is necessary to minimize the interval between the time at which the dark-image offset data are acquired and the time at which the radiation image is captured. If dark-image offset data are acquired before a radiation image is captured in each image capturing cycle, then the entire image capturing process is disrupted.